Coatings play a decisive role in substrates of all types in order to make possible their usability in various applications. In the case of plastics, and in particular plastic foils, such a refining step represents a weak point in many cases. One of the significant reasons for this is the low thermal stability of many plastics. Numerous coating technologies require preheated substrates in order to achieve the desired coating properties. Examples of this are thermochromic coatings, which are increasingly used in the field of so-called “smart windows”, barrier coatings, optical coatings, or transparent conductive coatings. For the group of transparent and electrically conductive metal oxide coatings (also called transparent conductive oxide, or TCO) on plastic foils, a variety of possible uses arise, such as, for example, in displays/advertisements, lights, as well as energy generation.
In the TCO coatings that are manufactured with a heated substrate, semi-crystalline or crystalline coatings result with a good electrical conductivity that are often also characterized by light transparency. By means of a thermic post-treatment of such coatings, coating defects can be annealed, as a result of which both the light transparency and the mobility of the charge carriers can be easily improved, and consequently also the electrical conductivity. In the TCO coatings that are manufactured with an unheated substrate, tempering the amorphous coating after depositing the coating can bring about a structural relaxation by local rearrangement, which increases the charge carrier concentration. Here, a crystallization process also occurs with nucleation and grain growth, which improves the mobility of the charge carriers.
By using a thermal post-treatment at high temperatures of approximately 300° C. to 600° C. on the applied TCO coatings, the electrical conductivity and the transparency of TCO coatings can be improved still further. Here, a lattice structure forms inside the coating, the ions assume energetically more favorable positions, and the number of scattering centers is reduced, which leads to a drastic reduction of the absorption or recombination. For a heat treatment, the heat temperature, the time the coated substrate stays in the heating system, and the optical properties of the coated substrate are decisive. What is negative with the heat treatment is that the heat absorption in a heating station is complicated in TCO coatings due to the higher reflection in the infrared spectral range. Furthermore, the required temperatures cannot be used on temperature-sensitive substrates either in the coating or in the post-treatment. For this reason, in TCO coatings that are applied to plastics, only a poor electrical conductivity and a poor transparency are often achieved.
In DE 10 2006 047 472 A1, it is disclosed that following a surface coating of thin functional coatings made of the most diverse materials for manufacturing electronic components, at least one thermal post-treatment by means of pulsed short-wavelength electromagnetic radiation, while avoiding an immediate heating of the entire substrate, leads to an annealing of structural defects. Here, the thermal post-treatment is affected by one or more gas discharge lamps, by at least one arc pulse, or by Xenon arc lamps. The pulse duration varies from 0.2 to 100 ms, and the pulse energy from 0.1 to 100 J/cm2. Furthermore, the pulse intensity, the pulse repetition frequency, the pulse shape, and the pulse duration of successive arc pulses must be adapted to the thickness of the coating to be treated and the mass of the substrate. The energy of the radiation can be increased by coupling the substrate to a larger mass.
A temperature treatment by laser radiation, a temperature treatment in a high-convection oven, or in a two-chamber oven after the deposition of a variety of transparent conductive metal oxides at low substrate temperatures, by means of which a deposited coating can be improved with respect to conductivity and transparency, is known from DE 10 2009 033 417 A1. Here, the temperature treatment occurs in ambient air in a continuous process using one or more laser diodes, Nd:YAG lasers or Yb:YAG/disk lasers. Due to the integration of a diffusion barrier, or an optical interference coating, the energy of the laser can be increased and the substrate is not excessively heated.
DE 10 2011 005 753 A1 shows that a pulse-like energy input by electromagnetic radiation as well as particle bombardment with high energy density in the atmosphere can affect an improvement of the properties of TCO coatings on plastic substrates, but limits the temperature range allowed for the substrate from room temperature to 180° C. Thus, the maximum processing temperature of the plastic, which should also not be exceeded during the coating of the substrate, is specified as the upper limit for the post-treatment. The heat treatment occurs along a line transverse to the transport direction of the substrate, and can be performed using lasers, gas discharge lamps (arc or halogen lamps) as well as an electron beam. The exposure time used here varies between 1 μs and 1 s. By using a separation coating between the substrate and the TCO coating made of a material having low thermal conductivity, the energy of the thermal post-treatment can be increased, which affects an additional improvement of the coating properties. In this case, it is disadvantageous that a heat treatment of a TCO coating, which should not heat a substrate to more than 180° C., often does not lead to the desired coating properties.